Monster Amp Meltdown

Peter S

Peter S

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#1
Hi Folks, my modified Crest CA-18 went out with a bang. Three or four almost gunshot blasts, I thought some electrolytics exploded. My fatal mistake may have been to install slower fans to reduce noise as the rack is located in the listening room. There have been rare occasions when the amp's full power has been demanded but I thought I could get away with quieter fans for home theatre use.
 

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Gepetto

Gepetto

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Peter S said:
Hi Folks, my modified Crest CA-18 went out with a bang. Three or four almost gunshot blasts, I thought some electrolytics exploded. My fatal mistake may have been to install slower fans to reduce noise as the rack is located in the listening room. There have been rare occasions when the amp's full power has been demanded but I thought I could get away with quieter fans for home theatre use.
Click to expand...
Special effects...
 
Peter S

Peter S

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So that wet spot was from leaking low rail PS caps and I'm assuming the fluid shorted out areas of the power supply PCB. The caps were measured with a basic Klein DVM and found to be about 30% below their rated capacitance but not shorted. I cleaned everything up and reassembled the amp (PITA!) The amp survived but I have a project in front of me;
I would like to rebuild this amp with more heat sink to eliminate fan cooling. The heat sinks never really got hot before but the filter caps did not survive the reduced air flow....Or they could just have been at the end og their life? The amp is a 1999 vintage and I have only owned it for 6 years.
 

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Peter S

Peter S

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All 12 filter caps show signs of bulging of the rubber vent plug. Replacing these caps with units that will fit on the PCB will be approx $700 CDN or more, and would be for 85 degree rated units. This is almost half the value of the amp. I am pondering an expansion of the cabinet to allow physically larger caps, maybe more capacitance, and addition passive heat sinks.
The amp uses four 18000 ufd 75V caps and eight 10000 ufd 95V caps. I was considering using fourteen 10000 ufd 100V caps, doubling up to replace the 18000 ufd's. what parameters should one look for, besides ufd and Voltage? If the cabinet is expanded and ventilated, would 105 C rated caps still be better or necessary? What is considered 'good' ripple current and ESR for this size cap?
 
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Peter S

Peter S

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And thanks 'wattsabundent' for the cap info and I think you're right about the oil! I have disassembled the amp now after testing the amp modules. There was another large puddle of oil-like fluid in the bottom of the chassis. It is somewhat conductive but I have not seen an electrolytic leak to that extent. Are oil filled capacitors better? The ones in this amp have clearly failed.
I am considering rotating and raising the modules so the fins are facing up. I could then install a bank of caps horizontally under each module. Then both sides of each module could be extended with aluminum plate which would accomodate reattachment to the chassis and more heat sink .
Any ideas/ pointers?
Thanks
Peter in Hillsdale Ontario
 

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wattsabundant

wattsabundant

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Oil filled caps are AC only and much smaller uf. As I look at the assembly, the caps vent into the board and there is no hole for venting. So the debris was forced down to the chassis. That means there is electrolyte on the chassis. You need to make sure there is none in the transformer.
 
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George S.

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Peter, Find the data sheet for the failed caps. There'll be lots of good info there. The manufacturers series # may be on the cap like Nichicon marks them or it may be color coding.
Once you know what you have, you can research the series and suitable subs.
 
Peter S

Peter S

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Hi George, having trouble locating any caps in the 35 mm x 120 mm size (or smaller) The reduced fan speed was still slightly audible during quiet scenes so I am getting serious about rebuilding the amp with passive cooling. Expanding the amp will give me some flexibilty for cap choices. I am assuming ripple current would be the next most critical factor, then hours of service?
 

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Peter S

Peter S

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Thanks Wattsabundant, the electrolyte did not make it to the transformer. I think it may have survived even if it did. The amp was a great performer, good S/N ratio and low distortion for a pro amp but the torroid transformer buzzed. After many experiments with DC blockers, I peeled the plastic wrapping and dipped the torroid 20 times in electrical varnish, allowing about a week between dips. The transformer is completely silent now.
 

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Gepetto

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Peter S said:
Hi George, having trouble locating any caps in the 35 mm x 120 mm size (or smaller) The reduced fan speed was still slightly audible during quiet scenes so I am getting serious about rebuilding the amp with passive cooling. Expanding the amp will give me some flexibilty for cap choices. I am assuming ripple current would be the next most critical factor, then hours of service?
Click to expand...
Seems Mouser has stock on what you need Peter... not cheap but in stock
 
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